1. Field of the Invention
The present invention relates to a semiconductor laser array and more particularly to a multi-wavelength semiconductor laser array.
2. Description of the Background Art
Solid state light sources (for example, an LED, a semiconductor laser, and an organic EL) make use of advantages such as high efficiency and long life, so that a wide range of their use has been developed. They are becoming to serve as light sources in place of the existing lamps, particularly in lighting devices and display devices. Among the solid state light sources, the semiconductor laser has high light-emitting efficiency and high compatibility with a projector, thereby being greatly expected to be as a next-generation light source.
On the other hand, a problem occurring upon the use of the semiconductor laser as the light source of the display device is a speckle noise. The speckle noise is a screen glare and caused by high coherence of the semiconductor laser. In particular, the screen glare seriously impairs video quality of the display device, so that measures to suppress the speckle noise are necessary. The measures can be taken to suppress the speckle noise in each step of a screen, an optical system in the display device, and the light source.
First, as the measure in the screen, the screen itself is vibrated to reduce the speckle noise. However, the mechanism is massive, so that it is unrealistic to apply it to a transportable projector in which the place of use is not limited. Moreover, as the measure in the optical system, the technique to insert a diffusion plate in an optical path has been known. This can reduce the speckle noise with the simple mechanism while light is greatly lost in the diffusion plate, so that more efficient measures are desired.
As the measure in the light source, the speckle noise can be reduced by increasing a spectral width of the semiconductor laser. This can be achieved by multiplexing wavelengths of the semiconductor laser having a narrow spectral width. As described above, the measures against the speckle noise in the screen and the optical system have many challenges, so that it is preferable to take measures in the light source.
As a technique to multiplex the wavelengths of the semiconductor laser, a plurality of semiconductor laser light sources that emit light beams having different wavelengths are prepared, and a projector is configured such that the same screen is irradiated with the light beams simultaneously. However, while this technique can easily obtain the effect of the speckle noise reduction, the mechanism becomes complicated and also the manufacturing cost is greatly constrained due to the requirement for the plurality of laser light source devices.
As the technique to solve this, a multi-wavelength semiconductor laser array emits light beams having a plurality of wavelengths in one semiconductor laser light source device. The semiconductor laser array includes a plurality of semiconductor laser elements in parallel, the semiconductor laser element being formed of a waveguide and a pair of reflection mirrors (namely, front end surface and rear end surface) sandwiching the waveguide. The multi-wavelength semiconductor laser array is a light source device in which the plurality of laser elements forming the semiconductor laser array emit light beams having different wavelengths from each other.
A monolithic semiconductor laser array is known as the multi-wavelength semiconductor laser array. For example, the monolithic semiconductor laser array disclosed in Japanese Patent Application Laid-Open No. 2000-11417 is the multi-wavelength semiconductor laser array including a first laser element having AlGaAs as an active layer and a second laser element having InGap as an active layer formed on the same GaAs substrate. This configuration enables the first and second laser elements to emit the light beams having the different wavelengths suitable for the composition of each active layer. In this manner, more than two different compositions of the active layers are formed on the same semiconductor substrate, which can achieve the multi-wavelength semiconductor laser.
Moreover, in the multi-wavelength semiconductor laser array disclosed in Japanese Patent Application Laid-Open No. 2008-4743, each waveguide of the semiconductor laser array is arranged densely in the central part of the array and sparsely in both ends to generate a temperature distribution in the array, to thereby multiplex the wavelengths. In other words, the laser elements as the heat source arranged densely in the central part of the array oscillates closer to the long-wavelength side than the laser elements at the end part of the array because a wavelength shift amount increases according to a temperature rise in the central part. In general, the semiconductor laser is known that as the temperature of the waveguide increases, the wavelength is shifted to the long-wavelength side.
However, to form the active layers having the different compositions on the same semiconductor substrate as disclosed in Japanese Patent Application Laid-Open No. 2000-11417, a complicated manufacturing method accompanied by the plurality of semiconductor crystal growth is needed. The complicated manufacturing method not only increases the number of steps but also causes quality degradation of the semiconductor crystals, which leads to impairment of productivity and reliability of the product.
Moreover, in the multi-wavelength laser array of Japanese Patent Application Laid-Open No. 2008-4743, the waveguides in the central part of the array are extremely densely arranged to generate the temperature distribution. Thus, when the semiconductor laser having a wide stripe and producing high output is formed, mutual optical interference occurs between the adjacent waveguides, which may result in an unstable operation of the laser.